Localized collection(s) of an undesired fluid or fluids may develop in a wellbore from various sources, and such collections or deposits may pose significant problems in wellbore operations. In general, an "undesired fluid" in a wellbore is any fluid (including mixtures thereof) which may interfere with a working fluid or with recovery of a production fluid such as oil and/or gas. For example, collection of an aqueous fluid or fluids, such as a heavy brine, in a hydrocarbon well prior to or during the course of production may hinder or reduce the production rate of the well, and may require expensive cleanout operations to remove the undesired fluid(s).
The problem of collection or deposition of undesired fluids is of particular concern in so-called "deviated" or curved wellbores, wellbores which depart significantly from vertical orientation. Particularly where the deviated wellbore is drilled with a downhole driving source, deviated wellbores commonly contain "dips" or depressions due principally to orientation shifts of the bit while drilling. The depressions, because of their horizontal component, provide locations or sites which are especially susceptible to collection of undesired fluid or fluids. These collections or "pools" of undesired fluids restrict the cross-section of the wellbore which is open to flow of the working or production fluid. While drilling fluid pressure is normally sufficient to maintain drilling mud movement during drilling operations, production fluid pressure may be significantly less, and the density differential between production fluid and the intruding liquid(s) can pose operational difficulties. Additionally, production fluids may not be miscible with a dense undesired fluid material, such as a heavy brine, and may not be able to displace or transport the undesired fluid.
A need, therefore, has existed for providing an effective "cleanout" means or method for elimination or removal of undesired fluid or fluids from wellbores. The invention addresses this need.